[英]How to remove rows in a matrix
I am faking a matrix as follows. 我伪造如下矩阵。
a = rep(1:4, each=2)
b = rep(0:1, times=4)
m = cbind(a, b)
m
a b
[1,] 1 0
[2,] 1 1
[3,] 2 0
[4,] 2 1
[5,] 3 0
[6,] 3 1
[7,] 4 0
[8,] 4 1
I need to remove any rows where a==4
and b==0
. 我需要删除a==4
和b==0
所有行。 I know how to select such rows: 我知道如何选择这样的行:
m[m[,1]==4 & m[,2]==0,]
a b
4 0
But I have no idea how to remove them. 但是我不知道如何删除它们。 I know that I can do m[-7,]
in this particular case. 我知道在这种情况下我可以做m[-7,]
。 But image that this is a huge matrix and I cannot visually inspect which rows meet the condition as a==4
and b==0
. 但是,这是一个巨大的矩阵,我无法直观地检查哪些行满足a==4
和b==0
。 I also tried this: 我也试过这个:
count=1:8
m.count = cbind(m, count)
m.count[m.count[,1]==4 & m.count[,2]==0,]
a b count
4 0 7
So this will automatically tells me which rows meet the condition and then I can use the index c=count[3]
(if only one row) or c=count[,3]
(if more than one row) and try m[-c,]
to get the right results. 因此,这将自动告诉我哪些行满足条件,然后可以使用索引c=count[3]
(如果只有一行)或c=count[,3]
(如果有多于一行)并尝试m[-c,]
以获得正确的结果。
But this solution is too lengthy. 但是这个解决方案太冗长了。 Does anyone know a easy way to solve the problem? 有谁知道解决问题的简便方法? I am expected to use "-"
in some smart ways to do it. 我应该以一些聪明的方式使用"-"
来做到这一点。
I think you just need ?which
我想你只需要?which
m <- m[-which(m[,1] == 4 & m[,2] == 0),]
Testing: 测试:
# Konrad's !
> system.time(rep(m[! (m[, 1] == 4 & m[, 2] == 0), ],10000000))
user system elapsed
0.264 0.094 0.357
# BB's which
> system.time(rep(m[-which(m[,1] == 4 & m[,2] == 0),],10000000))
user system elapsed
0.244 0.109 0.354
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